None Applicable
This invention relates generally to the field of microelectronics and more specifically to plastic packaging of microelectromechanical systems (MEMS) and integrated microelectromechanical systems (IMEMS) devices.
Examples of MEMS and IMEMS devices include airbag accelerometers, microengines, optical switches, gyroscopic devices, sensors, and actuators. For current commercially packaged MEMS and IMEMS components, the steps of packaging and testing can account for at least 70% of the cost. Also, the current low-yields of MEMS packaging are a xe2x80x9cshow-stopperxe2x80x9d for the eventual success of MEMS. Conventional electronic packaging methods, although expensive, are not presently adequate to carry these designs to useful applications with acceptable yields and reliability.
Current packaging methods first release the MEMS elements at the wafer scale, followed next by probe testing. Unfortunately, probed good MEMS are often lost in significant quantity due to damage during subsequent packaging steps. These subsequent steps include die separation, die attach to the package, wirebonding (or other interconnection methods), and sealing with hermetic or dust protection lids. Electrostatic effects, moisture, and rough handling can damage the fragile MEMS elements or wirebonds. There is a need to ruggedize the MEMS elements and wirebonds against damage during each of these packaging steps. Herein, the word xe2x80x9cwaferxe2x80x9d can include silicon; gallium arsinide (GaAs); or quartz wafers or substrates (e.g. for MEMS structures).
At some stage in the fabrication process the MEMS elements must be released (e.g. made functional) by etching away a sacrificial, protective layer of silicon dioxide or silicate glass that surrounds the MEMS elements. Typically, this is done at the wafer scale. A typical wet release procedure includes acid etching in hydrofluoric or hydrochloric acid, followed by rinsing and drying. Alternatively, dry plasma etching with chemically active ions, such as oxygen, chlorine, or fluorine ions, can be used. It is critical that the release process does not damage other features on the MEMS or IMEMS device, such as metal interconnects. Dry etching processes are generally less damaging to the fragile MEMS elements than wet processes, but can take more time to complete. A desirable goal is to postpone the release step until the last possible moment.
Many different types of microelectronic devices require an opening in the protective package that exposes a sensitive or active area to the surrounding environment. For example, MEMS elements (e.g. gears, hinges, levers, slides, mirrors, optical sensors, chemical sensors, etc.) must not be encapsulated in plastic because these free-standing structures must be able to move, rotate, etc. Also, MEMS packages can require optical access through a window to permit viewing and inspection for calibration and performance characterization of operating MEMS elements.
Likewise, microsensors that have chemically sensitive, pressure-sensitive, or temperature-sensitive areas (sometimes combined with IC""s, CMOS or Bipolar chips, etc.) must be freely exposed to the environment through an opening or openings in the plastic package. Finally, optically active microelectronic devices can require optical access through an opening in the plastic package (the package may, or may not, have a window attached across the opening). Examples of optically active devices include charge coupled devices (CCD), photocells, laser diodes, vertical cavity surface emitting lasers (VCSEL""s), and UV erasable programmable read-only memory chips (UV-EPROM""s). While some of these devices emit light, and while others receive light; both are considered to be xe2x80x9coptically activexe2x80x9d.
Therefore, some method of creating an opening in the plastic package is needed for these types of microelectronic devices.
Butler, et al. disclose a method of creating an opening in a plastic package using laser ablation to vaporize and cut a window through a 60 micron thick protective dielectric layer (e.g. 2 layers of Kapton film bonded with thermoplastic adhesive), thereby exposing the unreleased MEMS device or microsensor to the environment. See J. T. Butler, V. M. Bright, and J. H. Comtois, xe2x80x9cMultichip Module Packaging of Microelectromechanical Systemsxe2x80x9d, Sensors and Actuators A 70 (1998) 15-22. In the same reference, Butler, et al. disclose using a two-step process where the majority of the protective layer is removed by a high-power laser, followed by removing the remaining thin layer by using a low-power laser, or by using a plasma etching process. Then, after creating the opening, a conventional release etch can be performed to release the MEMS elements.
A problem with using an ablative process (e.g. laser ablation) to create an opening in a plastic package is potential damage to MEMS elements (or other sensitive surfaces) caused by overheating, and fracture or warping due to thermal stresses. Accurate control is required to prevent accidentally cutting through the protective layer and damaging the sensitive area below. Another potential problem is unwanted deposition of laser-ablated debris on to adjoining surfaces. Finally, the rate that material can be removed by laser ablation or dry plasma etching is generally much slower than that which can be achieved by non-ablative methods, including acid (e.g. wet) etching, mechanical machining, water jet cutting, vacuum or thermal processing methods.
In U.S. Pat. No. 5,897,338, Kaldenberg teaches a method for encapsulating an integrated semi-conductor circuit (die) comprising the following steps: a) mounting the semi-conductor circuit onto the surface of a lead frame, b) attaching connecting wires between the contact surfaces of the semi-conductor circuit and selected parts of the lead frame (bonding operation), c) by means of a mould producing a plastic housing which at least encapsulates the semi-conductor circuit, the supporting surface, the bonding wires and part of the lead frame, wherein d) the mould comprises an inwards extending section of which the end surface in the closed situation of the mould extends parallel to the free upper side of the integrated semi-conductor circuit at short distance thereof, and e) before closing the mould a layer of heat resistant deformable material in the form of a ring or a continuous layer is brought in between the upper side of the integrated semi-conductor circuit and the end surface of the inwards extending part, which layer not or hardly adheres to the plastic housing. The combination of the inwards extending part and the ring or layer of deformable material serve together to completely exclude any encapsulant from touching the surface of the IC, and serve to exclude any encapsulant from flowing into the volume directly above the designated surface area of the IC.
Kaldenberg does not discuss in the ""338 patent any application of his method to packaging of MEMS or IMEMS devices. However, if Kaldenberg""s method of the ""338 patent were to be applied to packaging of MEMS or IMEMS device, potential problems could include: damaging the MEMS elements if the inwards extending section is pushed down too hard; sticking of the deformable material to the MEMS surface; and sticking of the encapsulant to the inwards extending section.
The present invention differs from Kaldenberg""s ""338 patent in at least two ways. Firstly, in the present invention, the opening in the plastic package is created by physically removing (e.g. perforating) the encapsulant in a volume located above the sensitive area (e.g. xe2x80x9cdesignated area of the ICxe2x80x9d). Kaldenberg, on the other hand, never allows any of the flowable encapsulant to fill-in this volume because his patent teaches the step of inserting an excluding member through a hole in the top of the transfer mold frame prior to encapsulating.
Secondly, in the present invention, encapsulant is allowed to flow across the face of the designated area of the IC. Kaldenberg, on the other hand, teaches the use of a deformable ring or layer placed in contact with the surface of the IC to specifically prevent the flow of encapsulant across the face of the designated area. If Kaldenberg""s ""338 method were to be used without using the deformable ring or layer, then some encapsulant would flow across the face of the designated surface area of the IC; thereby defeating the purpose of Kaldenberg""s invention, namely, to provide complete access to the IC after molding, without the need to perform any additional material removal steps. Kaldenberg does not teach or suggest in the ""338 patent any method, or means for, removing encapsulant material which may have flowed across the face of the IC. Kaldenberg uses the deformable ring or layer to prevent such a flow from occurring, thereby obviating the need for removing encapsulant after the molding step.
In the present invention, a perforation is created in an electrically insulating material above the sensitive area. A commercial device currently exists for this purpose. Conventionally called a xe2x80x9cdecapsulatorxe2x80x9d, or xe2x80x9cjet etcherxe2x80x9d, such a device removes plastic material by directing a stream of heated acid etchant perpendicular to the surface being etched. A gasket can be used to confine the area being etched. Conventional etching solutions comprise fuming nitric acid, fuming sulfuric acid, or mixtures of the two acids. The etching solutions can be heated to 250xc2x0 C., and +/xe2x88x921% temperature control is provided. An example of a decapsulator device is the xe2x80x9cD Cap-Deltaxe2x80x9d dual acid system sold by BandG International, Santa Cruz, Calif., 95060.
Commercially available decapsulators are used for creating an opening in plastic packages to expose an integrated circuit (IC) contained therein. The etching solutions described above preferentially dissolve common plastic compounds, including epoxy resin, without attacking the underlying integrated circuit. Reasons for decapsulating the package include inspection, testing, failure analysis, and repair. Visual inspection can confirm the integrity of wirebond connections. Probe pins can be placed onto the exposed surface pads to test the operation of the IC (e.g. probe test). It is also desirable to test IC""s while the package is still mounted to the printed circuit board. Failure analysis also requires a method of opening up selected portions of a microelectronic device potted in plastic. Finally, damaged wirebond connections can be repaired, once the broken connection is exposed.
Commercially available decapsulators are not presently used to create an opening in plastic packages or covers for the purpose of exposing released (or unreleased) MEMS devices. They are also not presently used for the purpose of xe2x80x9cactivatingxe2x80x9d microsensors, e.g. by exposing the active sensor area freely to the surrounding environment. Decapsulators are also not presently used to create an opening for providing optical access to optically active areas on integrated circuits (e.g. CCD chips and UV-EPROMS).
What is desired is a method that provides a low-cost, high-yield, and high-capacity commercial packaging technique that releases the MEMS elements after covering or encapsulating the MEMS or IMEMS devices, delicate wirebonds, other electrical interconnections, and die attachments in a hardened plastic material. What is also desired is to minimize the number of processing steps that are performed after releasing the MEMS elements, in order to minimize the risk of damaging the fragile MEMS elements. It is also desired to use a material removal process that can create an opening in a plastic protective layer, without damaging the underlying sensitive area underneath (e.g. IC""s, MEMS elements, or sensor elements).
The present invention solves these problems by firstly creating an opening in the plastic protective layer by using a non-ablative material removal process, which exposes the sensitive area to the surrounding environment; and, secondly, releasing any MEMS elements by wet or dry etching the unreleased MEMS device with access provided by the opening created in the plastic cover.
Throughout this application, use of the word xe2x80x9cMEMSxe2x80x9d is intended to also include xe2x80x9cIMEMSxe2x80x9d devices, unless specifically stated otherwise. Likewise, the word xe2x80x9cplasticxe2x80x9d is intended to also broadly include any type of flowable dielectric composition.
The present method places the high-risk packaging steps ahead of the release of the fragile portions of the device. It also provides protection for the die in shipment between the molding house and the house that will release the MEMS elements and subsequently treat the surfaces. This packaging scheme is very inexpensive compared to placing a device in a discrete package. It permits one to obtain large numbers of functional devices for performance evaluation and debugging of designs. It lets the devices go through any rough handling or environments associated with packaging while still protected in encapsulating plastic. Adhesion problems, such as during die attach, are minimized because surfaces have not yet been treated with proprietary materials. This method can be used for inserting MEMS/IMEMS devices into existing commercial IC plastic packaging lines with minimal impact, thereby minimizing costs.